Alkali ion solid-state batteries, such as sodium metal halide batteries, are known and have been widely used in various applications. In a typical sodium metal halide battery, a solid-state electrolyte, such as β″-alumina solid electrolyte (BASE) or sodium super ion conductor (NASICON), is disposed between a molten sodium anode and a cathode, such as a metal halide (e.g., NiCl2). During discharge, sodium atoms in the anode donate electrons and migrate through the electrolyte to the cathode. To properly function, the electrolyte must be a good conductor of sodium ions, be a poor conductor of electrons, physically separate the anode and cathode materials, and have sufficient structural integrity to withstand the harsh environmental conditions during operation. These solid electrolyte devices are usually operated at high temperatures (around 300° C.), and materials of the electrodes are highly corrosive and reactive at these temperatures.
The electrolyte is fabricated into tubes, discs, or other shapes from sodium-conducting ceramic materials, such as BASE or NASICON. Conventional designs for sodium metal halide cells generally use a round or clover-leaf shaped tubular geometry in individually packaged containers. In conventional sodium ion conducting solid-state electrolyte designs, the structural integrity of each cell electrolyte depends solely on the solid electrolyte material itself. The electrolyte must be sufficiently thick, and the ceramic be sufficiently strong for the electrolyte to be self-supporting and to maintain its physical integrity. Typically, thickness is at least 1 mm, usually between about 1 and 2 mm, and fabrication requires prolonged sintering and conversion steps at high temperatures. This design results in high costs of materials and processing. In this regard, it is desirable to utilize thinner electrolyte layers to reduce the impedance of the cell (which has the benefit of yielding higher energy storage capacities, higher power outputs, and less heat production during operation).
Tubular type designs typically include a series of cylinders or cells connected to one another. The shape of the tubular design results in a high resistance (i.e., lower efficiency), poor power and energy densities (due to the need for a thicker cathode and limited/small active surface area) compared to other stack designs (such as planar type designs) due to an increased travel distance of the ionic species. The tubular shape also makes connecting individual cells with one another difficult and results in a less compact multi-cell structure.
Alternative planar type stack designs have been used that may have a higher efficiency than tubular style designs possibly due to a shorter electron travel distance. However, conventional planar type stack designs use large glass or brazed seals placed between each of the layers of the stack which can create a high degree of shear loading. When operated in the vertical position, the seal surfaces are in direct contact with the electrochemically active components of the cell (i.e., the corrosive materials of the stack, such as a molten salt) making them prone to accelerated corrosion and thus subsequent failure.
Additionally, as conventional planar cells are scaled to higher capacity, the BASE becomes more likely to fail due to bending and residual thermal stress during component and cell manufacturing and operation. Such planar cells may include glassing the BASE to an alpha alumina ring for structural support. Constructing a fully dense alumina ring with the requisite flatness and roundness makes the technology cost uncompetitive. Furthermore, production costs are also increased by the fact that the coefficient of expansion of any metal packaging employed must be perfectly matched to the BASE to avoid cell breakage as a result of placing the seals under a shear load.
As can be seen, various attempts have been made to address the above issues with conventional alkali metal halide batteries. Known alkali metal halide batteries fail to adequately address the sealing and scaling issues discussed above, and present safety issues if the content of the alkali metal, such as sodium, is uncontrollably released to the cathode compartment. Accordingly, improved alkali metal halide batteries and components thereof are desired.